What is a model, what makes a good one, and why does it matter that many security standards are models?

Question

Accepted Answer

A model is an incomplete representation of reality that makes certain aspects, mechanisms or relationships clear; a good model reduces complexity, explains, structures and orders. Many standards are models too, so they also show reality only incompletely.

* A model keeps what its purpose needs and drops the rest on purpose. The danger is forgetting what was dropped. *
The incompleteness is deliberate: a model leaves out what does not matter for its purpose, and that is exactly what makes it useful. A subway map is useless for walking distances and excellent for changing trains.
The consequence for cyber security architecture: when you apply a standard such as the ISO/OSI model or ISO 27001, remember that it covers some aspects of a real system well and ignores others. Treating a standard as a complete description of the system is how gaps go unnoticed.
Go deeper:

All models are wrong — Wikipedia — George Box's "all models are wrong, but some are useful": judge a model by its usefulness, not by completeness.
 Map–territory relation — Wikipedia — Korzybski's "the map is not the territory", the classic warning against mistaking a model for reality.